1. Field of the Invention
The present invention relates to an interchangeable lens system and, more particularly, to a so-called electronic mount system which permits the data required for various kinds of control to be communicated between a lens unit and a camera unit.
2. Description of the Related Art
With the recent development of video apparatus such as video tape recorders, it has been proposed to provide video cameras, camera-integrated video tape recorders and the like equipped with interchangeable lens systems, which have been popular in the field of conventional silver-salt cameras.
If an interchangeable lens system is applied to such a video tape recorder or the like, it is necessary to reliably control the drive system of a lens unit in accordance with a command sent from a camera unit. For this reason, it is necessary to transmit various kinds of information from the camera unit to the lens unit by means of a normalized and coded control signal so that the interchangeability of lens control information can be fully assured.
If such information transmission is applied to, for example, an exposure control (AE: auto-iris) device, it is possible to implement automatic exposure control of the type which is similar to that realized in a conventional camera-integrated video tape recorder having no interchangeable lens system.
FIG. 1 is a schematic block diagram showing one example of the arrangement of the exposure control system used in this kind of interchangeable lens system. Such an arrangement is disclosed in, for example, U.S. Pat. No. 4,959,728.
In FIG. 1, the right-hand side of the mount part MT shown by a middle chain line corresponds to a camera unit CM, while the left-hand side corresponds to a lens unit LS.
A subject image is formed on the imaging surface of an image sensor 3 of the camera unit CM via an iris 2 by a lens optical system 1. The subject image is photoelectrically converted by the image sensor 3 and is outputted as an image sensing signal. The image sensing signal outputted from the image sensor 3 is supplied to a camera signal processing circuit 4, where it is subjected to predetermined processing such as gamma conversion and a chrominance signal C and a luminance signal Y.gamma. are taken out as video signals. The chrominance signal C and the luminance signal Y.gamma. are passed through a camera encoder 5 of an NTSC type or the like, and are outputted from the camera unit CM in the form of a composite video signal or the like.
A luminance signal Y which is outputted from the camera signal processing circuit 4 is supplied to a detecting circuit 6, where it is subjected to detection. The detecting circuit 6 outputs the result as a control signal for controlling the iris 2 so that correct exposure which matches the state of luminance of a picture can always be obtained.
The control signal outputted from the detecting circuit 6 is converted into digital data by an A/D converter 7, and then fetched into a camera-unit controlling microcomputer (hereinafter referred to as a "camera microcomputer") 10. In the meantime, a reference value which is generated by a reference voltage generator 8 is similarly converted into digital data by the A/D converter 9, and then fetched into the camera microcomputer 10. In the camera microcomputer 10, the aforesaid control signal is normalized by using the reference value.
Arithmetic processing for this normalization is expressed by, for example, the following equation: ##EQU1## where Di: iris data,
Yc: level of the AE control signal inputted to the camera microcomputer, PA1 Yr: level of a signal Yc when the luminance signal is at its reference level, and PA1 Yb: level of the signal Yc when the image sensor is shielded from light.
An exposure control signal normalized through the above-described arithmetic operations is obtained.
Processing in the lens unit LS will be explained below. The exposure control signal is communicated from the camera microcomputer 10 to the lens unit LS over a data communication path 20. The communicated exposure control signal is fetched into a lens-unit controlling microcomputer (hereinafter referred to as a "lens microcomputer") 21, where it is subjected to serial-to-parallel conversion. The parallel output from the lens microcomputer 21 is converted into an analog control signal by a D/A converter 22. Then, the output of the D/A converter 22 is supplied to a driver 23, so that the iris 2 is driven.
FIGS. 2(a) and 2(b) are flowcharts showing the exposure control operation executed in the system of FIG. 1.
As shown in FIG. 2(a), processing on the camera-unit side includes the following steps.
Step #1: The output of the detecting circuit 6 is fetched into the camera microcomputer 10 from the A/D converter 7.
Step #2: A reference level is fetched into the camera microcomputer 10 from the A/D converter 9.
Step #3: The camera microcomputer 10 computes iris data Di on the basis of the above-described equation.
Step #4: The process waits until the input timing of a V sync signal (a video vertical synchronizing signal) which follows after a predetermined number of V sync signals has been inputted.
Step #5: A chip select signal is set.
Step #6: The camera microcomputer 10 effects parallel-to-serial conversion of the iris data and transmits the result from the camera unit CM to the lens unit LS by serial communication.
Step #7: The chip select signal is reset to bring the communication to an end.
Through the above-described operation, the transmission of the iris data from the camera unit CM to the lens unit LS is completed.
Processing by the lens microcomputer 21 on the lens-unit side shown in FIG. 2(b) will now be explained.
Step #8: It is determined whether the chip select signal indicative of the start of communication has been inputted.
Step #9: The lens microcomputer 21 fetches the iris data by effecting serial-to-parallel conversion of the serial communicated data.
Step #10: The received iris data is sent to the D/A converter 22.
Step #11: The iris 2 is driven on the basis of the iris control signal outputted from the D/A converter 22.
Communication of control information between the camera unit and the lens unit is performed in the above-described manner, whereby iris control similar to a conventional one is provided.
As is apparent from the foregoing description, no particular problem will be encountered during the automatic exposure adjustment of the camera-integrated video tape recorder having the aforesaid interchangeable lens system.
It is to be noted, however, that the exposure control of the above-described interchangeable lens system is similar to that of a conventional system using no interchangeable lens system. In other words, the above-described interchangeable lens system is arranged to supply the amount of displacement relative to the current state of the iris and performs relative iris control which allows for no aperture value.
A transient operation occurring when the quantity of light varies will be considered here. According to the above-described iris control, the following steps are repeated:
Detection output varies.fwdarw.Iris driving data is outputted.fwdarw.Communication is performed.fwdarw.Iris is driven.fwdarw.Quantity of light varies.fwdarw.Detection output varies.
During this process, if the loop gain of an exposure control system is high, the iris may be excessively controlled to cause a transient variation in the state of exposure, with the result that an imperfect image may occur immediately before the image stabilizes. Oscillation may also occur under particularly unfavorable conditions.
If the exposure control system is stabilized to such an extent that no overshoot occurs in the control system, the speed of response will lower and relatively slow control will result.
The above-described transient operation of the exposure control system also results from the duration of time required for data communication which is needed in the interchangeable lens system.
Research and development which have been conducted since U.S. Pat. No. 4,959,728 has revealed that there is a room for improvement in the above-described respects.